BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to a method of recovering sulfuric acid from
a mixture of sugars and concentrated sulfuric acid, and more particularly, to a method
of recovering sulfuric acid from the hydrolyzate product obtained from the a(-- hydrolysis
of biomass.
[0002] As a result of the continual depletion of nonrenewable energy sources and the rapid
escalation of energy prices, various energy conservation measures have been encouraged
and alternative energy sources have been proposed and studied. Among the proposed
alternative enegy sources, current research has focused heavily on the use of biomass
as a viable energy source.
[0003] Biomass is composed of three major materials: cellulose, hemicellulose and lignin
in ratios of roughly 4:3:3. This composition enables biomass to be a versatile alternative
energy source since cellulose is a polymer of hexose rings and hemicellulose is a
polymer composed of hexose and pentose rings. These polymer chains may be broken and
converted to sugars and other chemicals through various chemical, microbial or fermentation
processes.
[0004] The principal sources of lignocellulosic biomass are agricultural crops, agricultural
residues, forest products, and municipal waste. Biomass is particularly attractive
as an alternative energy source since it is available in large quantities and is renewable.
It can also be converted to a variety of chemicals and its conversion need not create
air pollution problems. Instead, such conversion can assist in alleviating municipal
waste problems.
[0005] To optimize the conversion of biomass to energy producing fuels and other valuable
chemicals, it is necessary to fractionate the crude biomass to the sugar monomers:
glucose and xylose. The most common method used in accomplishing this conversion is
acid hydrolysis. In general, the acid hydrolysis of biomass requires either high temperatures
and dilute acid or high acid concentrations and low temperatures to obtain acceptably
high sugar yields. The high temperature/dilute acid process has the advantage of not
requiring acid recovery, because of the relatively inexpensive dilute acid being utilized.
However, this process has the disadvantage of suffering low sugar yields caused by
the degradation of sugars at the high temperatures needed.
[0006] The high acid concentration/low temperature process produces high sugar yields, but
the economic success of the process requires acid recovery. Accordingly, it is recognized
that for the conversion of biomass to sugars and other usable chemicals through the
use of concentrated acids to be economically feasible, the process must include an
efficient acid recovery procedure. Typically, dilute acid processes involve acid concentrations
of 5% or less, while concentrated acid processes involve acid concentrations of 10%
or more.
[0007] The principal method of treating the hydrolyzate solution containing sulfuric acid
and sugars has been neutralization with lime and removing the sulfate salts as gypsum.
A more economical method of recovery is by diffusion dialysis or electrodialysis.
[0008] The Hokkaido process, commercialized in Japan in the early 1960's, reportedly utilized
this technology. The disadvantages of this method include the high capital costs for
the equipment and the difficulties associated with both obtaining complete acid/sugar
separation and a high acid concentration.
[0009] Accordingly, there is a need in the art for a method of recovering concentrated sulfuric
acid from a sugar/sulfuric acid product obtained from the acid hydrolysis of biomass
or other cellulose materials. A further need is a method to separate concentrated
sulfuric acid and still permit subsequent recovery of the sugars.
SUMMARY OF THE INVENTION
[0010] In accordance with the present invention, concentrated sulfuric acid can be recovered
from the sugar/sulfuric acid mixture obtained from the acid hydrolysis of biomass
through a proce involving solvent extraction. Specifically, the process of the present
invention involves contacting the hydrolyzate solution in an extraction device with
any of the C
4-C
7 alcohols, mixtures of these alcohols, or solutions in which these alcohols are a
major component. This initial extraction results in the separation of the hydrolyzate
into an acid-rich extract phase and a glucose-rich raffinate phase. In accordance
with the preferred procedure, the glucose-rich phase, which still contains a small
amount of residual sulfuric acid, is neutralized by the addition of lime or other
similar material to remove the sulfuric acid. This yields a glucose solution for fermentation
or processing in another manner.
[0011] The acid-rich extract phase is then extracted with another solvent such as benzene,
carbon tetrachloride or toluene to remove the alcohol. The sulfuric acid stream from
this second extraction is available for reuse in the hydrolysis of other cellulosic
materials. The solvent added in the second extraction is separated from the C
4-C
7 alcohol (the first extraction agent) by distillation. The separated C
4-C
7 alcohol is then recycled for use in the first extraction procedure described above,
while the separated second extraction solvent is recycled for use in the second extraction
procedure.
[0012] Accordingly, it is the object of the present invention to provide a method of recovering
sulfuric acid from a sugar/sulfuric acid mixture.
[0013] Another object of the present invention is to provide an efficient and economical
method of recovering sulfuric acid from a sugar/sulfuric acid mixture obtained from
the acid hydrolysis of biomass.
[0014] A further object of the present invention is to provide a method of recovering sulfuric
acid using an extraction procedure.
[0015] These and other objects of the present invention will become apparent with reference
to the drawings, the description of the preferred method and the appended claims.
. DESCRIPTION OF THE DRAWINGS
[0016]
Figure 1 is a schematic diagram of the solvent extraction sulfuric acid recovery process
in accordance with the present invention.
Figure 2 and 3 are ternary phase diagrams showing phase equilibrium data for the preferred
Solvent I, heptanol, and Solvent II, benzene, respectively.
DESCRIPTION OF THE PREFERRED METHOD
[0017] While the preferred method of the present invention has application to any procedure
in which it is desired to recover concentrated sulfuric acid from a stream containing
concentrated sulfuric acid and sugars, it has particular application in a process
for recovering concentrated sulfuric acid from the hydrolyzate produced from the acid
hydrolysis of biomass. There are a number of methods utilized in the acid hydrolysis
of biomass or other cellulose-containing materials for producing a hydrolyzate having
a composition of sugars, water and concentrated sulfuric acid. Those for which the
present invention would have the greatest applicability, however, are those involving
low temperatures and high acid concentrations.
[0018] In a typical process involving the acid hydrolysis of oiomass using concentrated
sulfuric acid, the acid concentration is generally about 20% or greater. The method
of the present invention is applicable to these concentrations of sulfuric acid and
additionally, is applicable for the recovery of sulfuric acid as low as about 10%.
Thus, for purposes of the present method, a concentrated acid will be considered as
any concentration greater than about 10%.
[0019] With reference to the schematic diagram illustrated in Figure 1, the preferred procedure
involves exposing the hydrolyzate provided via stream 1 to an extrartion solvent ("Solvent
I") in the extraction column 21. The Solvent I is provided via stream 2. The hydrolyzate
is the product of the acid hydrolysis of biomass and is a composition containing primarily
sulfuric acid (H
2SO
4), sugars in the form of glucose and xylose, and water. The percentage of each of
these compositions will depend upon the concentration of acid utilized in the acid
hydrolysis procedure and also on the efficiency of such procedure in converting cellulose
and related materials to sugars. Normally, however, it is contemplated that the hydrolyzate
stream 1 will contain a sugar component between about 5% and 20%. It is also contemplated
that the concentration of sulfuric acid in this hydrolyzate will be between about
20% and 75% on a glucose-free basis, although it is possible for the sulfuric acid
concentrations to be outside this range. For purposes of describing the preferred
method, the ratio of the flow of hydrolyzate in stream 1 to Solvent I in stream 2
is approximately 1:3. Stream 1 contains approximately 55% sulfuric acid (61% on a
sugar-free basis), 40.5% water and 4.5% sugars.
[0020] The hydrolyzate is directed to the countercurrent solvent extraction column 21 where
the hydrolyzate is extracted with any one of the
C4-
C7 alcohols, mixtures of such alcohols or other solvent mixtures in which at least one
of such alcohols is a major component. In the preferred procedure, the hydrolyzate
solution which is provided to the extraction column 21 through stream 1 contains 61%
by weight sulfuric acid on a glucose or sugar-free basis together with portions of
sugars (glucose, xylose or both) and water. Normally the sugar concentration will
be expected to be between 5% and 20% by weight on an acid-free basis. In the preferred
system, however, stream 1 will contain 40.5% water and 4.5% sugars.
[0021] In the extraction column the sulfuric acid will be preferentially extracted from
the glucose/water solution. In the procedure of the present invention, it is contemplated
that the extraction column 21 can be any conventional extraction column, either countercurrent
or co-current. However, a countercurrent column is preferred. The column 21 must be
constructed of materials which are resistant to the acid environment of the materials
which will come in contact with the column. An acceptable extraction column is a Karr
extraction column.
[0022] In the extraction column 21, the hydrolyzate is fed from the bottom through the stream
1 while the extraction solvent is fed from the top through the stream 2 so that the
two streams pass through the extraction column 21 countercurrently. During exposure
and mixing of these two streams in the column 21, sulfuric acid is preferentially
extracted by the solvent and exits from the column 21 in an acid-rich extract phase
through stream 4. A glucose-rich raffinate phase is removed overhead from the extraction
column 21 through stream 3.
[0023] It should be noted that in the preferred procedure, the extraction column 21 is operated
at atmospheric pressure and at temperatures slightly above room temperature (i.e.
25°-35°C). It is contemplated, however, that the procedure can be carried out at other
temperatures and pressures as well. As with any extraction process, the efficiency
of the extracting procedure can normally be increased by increasing the number of
stages in the extraction column. Although an extraction column with any number of
stages will be effective to obtain the benefits of the present invention, an extraction
column having 10 to 20 stages is preferred. It also be noted that preferably the hydrolyzate
and the extraction solvent are fed through streams 1 and 2, respectively, in a ratio
of approximately 1:3.
[0024] The phase behavior of the hydrolyzate, when extracted with an appropriate extraction
solvent, is determined by the ability of the solvent to preferentially extract sulfuric
acid from the hydrolyzate. A common way of obtaining information regarding this ability
is by developing phase equilibrium data for the system and displaying such data in
a ternary phase diagram. The inventors have done this for certain of the C
4-C
7 alcohols including heptanol, hexanol, n-amyl alcohol, iso-amyl alcohol and isobutanol.
Heptanol appears to have the ability to extract the largest concentrations of sulfuric
acid, and is therefore preferred. Using heptanol as the extracting solvent, virtually
all of the sulfuric acid can be extracted from hydrolyzate solutions containing concentrations
of sulfuric acid as high as 73%. The other C
4-C
7 alcohols exhibited a more limited ability to extract high concentrations of sulfuric
acid, but sufficient ability to be considered for most hydrolyzate solutions. Specifically,
hexanol was shown to have the ability to extract virtually all of the sulfuric acid
from solutions containing concentrations of sulfuric acid as high as 62%, n-amyl alcohol
and iso-amyl alcohol each as high as 43% and isobutanol as high as 33%.
[0025] In the experimental procedure for developing the above mentioned phase equilibrium
data, a predetermined quantity of a synthetic hydrolyzate solution was brought into
contact with differing amounts of various extracting solvents and then shaken vigorously
in a separatory funnel. After allowing the mixture to separate into two equilibrium
phases, the top being the extract phase and the bottom being the raffinate phase,
the components of each phase were measured. The following Table 1 is a summary of
this data for the preferred solvent, heptanol. Figure 2 reflects this same data in
the form of a ternary phase diagram.
[0026] In addition to the C
4-c
7 alcohols, other solvent systems containing these alcohols as a major component may
be used as well. For example, these alcohols have been mixed together and have also
been combined with various other solvents, with the resulting mixtures showing effectiveness
as extracting solvents in the process of the present invention. Although it is contemplated
that these other solvents can be present and effective in various concentrations,
the C
4-C
7 alcohols must constitute a major portion of the extracting solvent system. Accordingly,
the extracting solvent system should have a C
4-C
7 concentration of at least 50% and preferably at least 75%.
[0027] In the preferred procedure, the sugar-rich raffinate from extraction column 21 which
exits through stream 3 contains approximately 90% water, 10% sugar and negligible
amounts of alcohol and sulfuric acid. This stream 3, in the preferred process, is
neutralized by the addition of lime or other similar material and fed to a fermentation
system to produce chemicals or used to recover crystalline glucose.
[0028] The acid-rich extract phase in stream 4 contains the solvent introduced in stream
2, along with the extracted sulfuric acid and water. In the preferred method using
heptanol, stream 4 comprises approximately all of the sulfuric acid originally in
the hydrolyzate. In the preferred procedure, a material balance shows that stream
4 contains approximately 79.4% Solvent I, 14.5% sulfuric acid, 5.3% water and the
remainder sugar.
[0029] This stream 4 is then directed to a second extraction column 22 for the purpose of
separating sulfuric acid from Solvent I. It should be noted that sulfuric acid is
not volatile; thus it cannot be effectively separated from Solvent I or the stream
4 by distillation techniques. In the extraction column 22, a second extraction solvent
("Solvent II") is introduced via stream 5 to preferentially extract Solvent I from
the sulfuric acid. Included as preferred solvents for this purpose are benzene, toluene
and carbon tetrachloride, although it is contemplated that various other solvents
with an affinity for organics, but not water, may be used as well. These include solvents
such as chloroform and ether. The extraction column 22, like the column 21, can be
of various constructions, although the preferred procedure contemplates a countercurrent
column.
[0030] The specific behavior of various extracting solvents in the extraction column 22
can be determined by developing equilibrium phase data in the same manner as described
above with respect to the solvents for the extraction column 21. For example, various
amounts of the second extracting solvent were combined with a solution of water, H
2SO
4 and a C
4-C
7 alcohol (for each of several H
2S0
4 concentrations). This mixture was then shaken and allowed to separate. Because of
the properties and.relationship of the solvents, none of Solvent II remained in the
raffinate phase. In other words, of the concentrations involved, 100% of Solvent II
remained in the extract phase. The concentrations in each of the phases was determined
and then plotted on a ternary phase diagram. Figure 3 shows such a diagram for benzene
with iso-amyl alcohol being utilized as Solvent I.
[0031] The following Table 2 is the experimental data used to develop the diagram of Figure
3. In both Figure 3 and Table 2, the raffinate phase data has been omitted because
it contains no Solvent II.
Following the development of such data, it was determined chat the three solvents
identified above, namely, benzene, toluene and carbon tetrachloride, are preferred.
[0032] The acid-rich raffinate from the extraction column 22, which exits via stream 6,
contains essentially all of the sulfuric acid and water fed in stream 4. This is the
recovered sulfuric acid which is recycled for use in the main acid hydrolysis process.
To the extent small amounts of sugar remain in stream
6, it is not lost since it is recycled back through the system along with the H
2S0
4. The extract phase from this second extraction which exits via stream 7 contains
both Solvents I and II initially introduced via streams 2 and 5, respectively, with
a very small amount of sulfuric acid. The Solvents I and II in stream 7 are finally
separated by distillation in a conventional distillation unit 23 to yield streams
8, and 9 for recycle. Stream 8 which contains the Solvent II (i.e. benzene, toluene,
carbon tetrachloride, or other organic solvent) is recycled to stream 5, while stream
9, which contains the Solvent I (i.e. C
4-C
7 alcohols) is recycled to stream 2. To the extent stream 9 also contains small amounts
of sugar and H
2S0
4, these are not lost since they are recycled back through the system along with the
recovered Solvent I. In the preferred method, vacuum distillation is employed in the
distillation unit 23 to yield streams 8 and 9 to prevent the reaction of alcohols
with residual sulfuric acid. Vacuum distillation allows the separation of solvents
at reduced temperatures in comparison to the higher temperature atmospheric distillation.
[0033] Although the description of the preferred procedure has been quite specific, it is
contemplated that various modifications could be made without deviating from the spirit
of the present invention. Accordingly, it is intended that the scope of the present
invention be dictated by the appended claims rather than by the description of the
preferred procedure.
1. A method of recovering concentrated sulfuric acid from the crude product obtained
from the acid hydrolysis of a cellulose-containing material, said method comprising
the steps of:
contacting the crude product with a first solvent comprising one or more of the C4-C7 alcohols as the major component and separating said product into a sulfuric acid
enriched phase and a sulfuric acid depleted phase: and
separating and recovering the sulfuric acid from the sulfuric acid enriched phase.
2. The method of claim 1 wherein said contacting step is carried out in a countercurrent
extraction column.
3. The method of claim 2 wherein the crude product is supplied to a first end of said
extraction column and said solvent is supplied to a second end of said extraction
column and wherein said sulfuric acid enriched phase is removed from the first end
of said extraction column and said sulfuric acid depleted phase is removed from the
second end of said extraction column.
4. The method of claim 1 wherein said crude product is separated into a sulfuric acid
enriched phase and a sulfuric acid depleted phase by a first extraction procedure.
5. The method of claim 4 wherein said first solvent is comprised of a mixture of two
or more of the C 4-C7 alcohols.
6. The method of claim 4 wherein said first solvent is comprised of at least 50 percent,
preferably at least 75 percent of the C4-C7 alcohols.
7. The method of claim 6 wherein said first solvent is comprised essentially of the
C4-C7 alcohols.
8. The method of claim 1 including separating and recovering the sulfuric acid from
the sulfuric acid enriched phase by a second extraction procedure.
9. The method of claim 8 wherein said second extraction procedure utilizes a second
solvent with an affinity for organics, but not for water.
10. The method of claim 5 wherein said second extraction procedure utilizes a second
solvent selected from the group consisting of benzene, toluene and carbon tetrachloride.
11. The method of claim 9 wherein said second extraction procedure separates said
sulfuric acid enriched phase into a raffinate phase comprising essentially sulfuric
acid and water and an extract phase comprising essentially said first and second solvents
and further including the step of separating and recovering said first and second
solvents from each other by distillation.
12. The method of claim 11 wherein said distillation is carried out under vacuum.